The spectral resolution of a concave grating spectrometer generally degrades with an increase in numerical aperture (NA) of grating illumination and spectrometer input slit size. The resolution is a convolution of the dispersion associated with the concave grating groove density, entrance slit size, and optical aberration from the grating, which is affected by the extent of illumination on the grating. A lower dispersion value increases the distance between wavelengths and facilitates higher spectral resolution. A low dispersion spectrometer, usually requires a long distance from the input or entrance slit to the grating and high groove density of the grating. While this provides higher resolution, the associated size of the spectrometer will be bulky. In addition, spectral coverage will be limited due to high groove density.
For spectrometers with compact size (such as less than 100 mm focal length) and broad spectral coverage (such as UV-VIS-NIR wavelengths from 200 nm to 1000 nm), resolution to resolve spectral lines is limited. As such, compromises are typically required when selecting instrument performance parameters for a particular application to achieve desired throughput, resolution, spectral coverage, etc. Known strategies for providing a finer resolution of a concave grating spectrometer include selection or modification of alternative spectrometer components including the diffraction grating and/or detector, for example. Scientific-grade spectrophotometers, which typically cover wavelengths from 200 nm to 1100 nm for general purpose applications, spectrometer resolution less than 2 nm is often required. Thus, these systems rarely utilize compact concave grating-based spectrometers due to the challenge in achieving resolution of less than 2 nm.
For applications that require a finer spectral resolution and compact size, such as an industrial spectrophotometer and fluorometer, a Czerny-Turner spectrometer is commonly used. To achieve a selectable resolution in a Czerny-Turner spectrometer, the input slit size is typically varied by a motorized slit opening. To maintain a good spectral resolution, typical Czerny-Turner spectrometers are designed with an F/# below F/4. Further reduction in the grating illumination area to improve spectrometer resolution is avoided because the system throughput loss will be unacceptable for most applications.